http://www.potsaa.com/hull-brown/

hull is fiberglass with brown cement like substance?
Help! I'm cleaning the Buttom of my boat (hull) and I can not to get the brown cement-like substance off. What can I do
I suspect that your boat is in water for a long period in the muddy water, then you took it out, and do not wash off the brown immediately. Right? This pipe down and keep it moist during few hours, then get a brush with soap as a detergent for washing dishes, and get there, keeping the wet dark sides. that does not work, take a car wash and try to use the high pressure Nossel. Remember to always try to clean your boat to the place that you go out water. Regards, Dan
Cat Empire – Will Hull Brown solo – complete legend!!